1. Field of the Invention
Embodiments disclosed herein relate generally to hardmetal composite materials used in drill bits.
2. Background Art
Conventional drilling systems used in the oil and gas and mining industries to drill wellbores through earth formations include a drilling rig used to turn a drill string which extends downward into a well bore. A drill bit is typically connected to the distal end of the drill string and is designed to break up earth formation in its path when rotated under an applied load. Typically, drilling fluid or air is pumped through the drill pipe and drill bit to move cuttings away from the bit during drilling and up an annulus formed between the drill string and the borehole wall.
Drill bits used to drill wellbores through earth formations generally are made within one of two broad categories of bit structures. Drill bits in the first category are generally known as “fixed cutter” or “drag” bits, which usually include a bit body formed from steel or another high strength matrix material (e.g., tungsten carbide) and a plurality of cutting elements disposed at selected positions about the bit body. The cutting elements are typically formed with a diamond or other ultrahard cutting layer disposed on a tungsten carbide substrate.
Drill bits of the second category are typically referred to as “roller cone” bits, which include a bit body having one or more roller cones rotatably mounted to the bit body. The bit body is typically formed from steel or another high strength material. The roller cones are also typically formed from steel or other high strength material and include a plurality of cutting elements disposed at selected positions about the cones. The cutting elements may be formed from the same base material as is the cone. These bits are typically referred to as “milled tooth” bits. Other roller cone bits include “insert” cutting elements that are press (interference) fit into holes formed and/or machined into the roller cones. The inserts may be formed from, for example, tungsten carbide, natural or synthetic diamond, boron nitride, or any one or combination of hard or superhard materials.
Most cutting elements include a substrate of tungsten carbide, a hard material, interspersed with a binder component, preferably cobalt, which binds the tungsten carbide particles together. When used in drilling earth formations, the primary contact between the tungsten carbide cutting element and the earth formation being drilled is the outer end of the cutting element. Breakage or wear of the inserts, among other factors, limits the longevity of a drill bit. Inserts used with a drill bit are generally subjected to high wear loads from contact with a borehole wall, as well as high stresses due to bending and impact loads from contact with a borehole bottom. The high wear loads can also cause thermal fatigue in the inserts, which initiates surface cracks on the inserts. These cracks are further propagated by a mechanical fatigue mechanism that is caused by the cyclical bending stresses and/or impact loads applied to the inserts. Fatigue cracks may result in chipping, breakage and failure of inserts. Tungsten carbide cutting elements may also fail by excessive wear because of their softness.
Many different types of tungsten carbides are known based on their different chemical compositions and physical structure. Of the various types of tungsten carbide commonly used in drill bits, cemented tungsten carbide (also known as sintered tungsten carbide) is typically used in cutting elements for drill bits. Cemented tungsten carbide refers to a material formed by mixing particles of tungsten carbide, typically monotungsten carbide, and particles of cobalt or other iron group metal, and sintering the mixture. In a typical process for making cemented tungsten carbide, small tungsten carbide particles and cobalt particles are vigorously mixed with a small amount of organic wax which serves as a temporary binder. An organic solvent may be used to promote uniform mixing. The mixture may be then be pressed into solid bodies often referred to as green compacts. Such green compacts are then heated in a vacuum furnace to first evaporate the wax and then to a temperature near the melting point of cobalt (or the like) to cause the tungsten carbide particles to be bonded together by the metallic phase.
In general, cemented tungsten carbide grades are primarily made in consideration of two factors that influence the lifetime of a tungsten carbide insert: wear resistance and toughness. As a result, conventional tungsten carbide grades used for cutting elements of downhole drilling tools have cobalt contents of 6% to 16% by weight and tungsten carbide “relative” particle size numbers of 3 to 6 (which equates to an average tungsten carbide grain sizes of less than 3.0 microns (μm), as measured by the ASTM E-112 method). These conventional grades typically have a Rockwell A hardness of between 85 and 91 Ra, a fracture toughness below 17 ksi(in)0.5 (as measured by the ASTM B-771 method) and a wear number between 1.8 to 5.0 (as measured by the ASTM B-611 method). In particular, these grades are widely used for inserts forming interior rows on roller cone bits.
For a WC/Co system, it is typically observed that the wear resistance increases as the grain size of tungsten carbide or the cobalt content decreases. On the other hand, the fracture toughness increases with larger grains of tungsten carbide and greater percentages of cobalt. Thus, fracture toughness and wear resistance (i.e., hardness) tend to be inversely related: as the grain size or the cobalt content is decreased to improve wear resistance of a specimen, its fracture toughness will decrease, and vice versa.
Due to this inverse relationship between fracture toughness and wear resistance, the grain size of tungsten carbide and cobalt content are selected to obtain desired wear resistance and toughness. For example, a higher cobalt content and larger WC grains are used when a higher toughness is required, whereas a lower cobalt content and smaller WC grain are used when a better wear resistance is desired. The inverse relationship between toughness and wear for carbide composites having varying particle size and cobalt content is shown in FIG. 1.
Gage row inserts are often selected to have a higher wear number than interior row inserts because it is generally believed that gage inserts need higher wear resistance due to the large amount of borehole wall contact they encounter during drilling. As a result, the toughness of gage inserts is typically sacrificed to gain wear resistance. However, this practice improperly assumes that the rock to be drilled by the gage inserts generally has the same properties in every application. In many applications, this is not the case and this practice has led to the breakage of gage inserts with the interior rows still intact.
For example, when drilling softer formations, such as carbonates, the wear resistance of inserts is not the major concern because these formations are not very abrasive. Rather, resistance to thermal fatigue and heat checking has been found to be the primary concerns that result in premature cracking and breakage of inserts. This occurs because the tungsten carbide inserts of a rock bit are subjected to high wear loads from contact with a borehole wall, as well as high stresses due to bending and impact loads from contact with the borehole bottom. These high wear loads can lead to thermal fatigue of the inserts which, in turn, leads to the initiation of surface cracks (referred to as heat checking) on inserts. These surface cracks are then propagated by a mechanical fatigue mechanism caused by the cyclical bending stresses and/or impact loads applied to the inserts during drilling. The result is chipping, breakage, and/or failure of inserts which shortens the useful life of the drill bit.
In particular for roller cone drill bits, inserts that cut the corner of a borehole bottom are often subjected to the greatest amounts of thermal fatigue due to heat generation on the inserts from a heavy frictional loading component produced as the inserts engage the borehole wall and slide into their bottom-most crushing position. As the cone rotates, the inserts retract from the borehole wall and are quickly cooled by circulating drilling fluid. This repetitive heating and cooling cycle can lead to the initiation of surface cracks on the inserts (i.e., heat checking). These cracks are then propagated through the body of the insert as the insert repeatedly impacts the borehole wall and high stresses develop.
The time required to progress from heat checking to chipping, and eventually, to breakage of inserts depends upon several factors including the formation type, rotation speed of the bit, and applied weight on bit. In many applications, especially those involving higher rotational speeds and/or higher weights on bit, thermal fatigue and heat checking of inserts are issues that have not been adequately addressed. Consequently, inserts made of standard tungsten carbide grades have been found to frequently fail in these applications.
To help reduce insert failures caused by thermal fatigue and heat checking, coarser grain carbide grades have been proposed for cutting elements of drill bits. Examples of grades proposed are further described in U.S. Pat. No. 6,197,084, U.S. Pat. No. 6,655,478, U.S. Pat. No. 7,017,677, U.S. Pat. No. 7,036,614, U.S. Pat. No. 7,128,773, and U.S. Publication No. 2004/0140133 A1, which are all assigned to the assignee of the present invention and incorporated herein by reference. These grades comprise coarse carbide grains having average grain sizes greater than 3.0 μm and binder contents of 6 to 16% by weight. Inserts formed from these composite materials have been found to exhibit higher fracture toughness and adequate wear resistance for many drilling applications. These inserts have been shown to result in improved performance and/or longevity when compared to inserts formed of conventional carbide grades. In particular, coarser grain composites have been found to be particularly useful in reducing gage carbide failures due to heat checking. However, high wear resistance is sacrificed.
Accordingly, there exists a continuing need for improvements in materials that possess both increased toughness and wear resistance.